This invention relates generally to centrifugal pumps, and, more particularly, to an improved method and apparatus for measuring the pressure at the suction side of a centrifugal pump.
As is known, a centrifugal pump has a wheel fitted with vanes and known as an impeller. The impeller imparts motion to the fluid which is directed through the pump. A centrifugal pump provides a relatively steady fluid flow. The pressure for achieving the required head is produced by centrifugal acceleration of the fluid in the rotating impeller. The fluid flows axially towards the impeller, is deflected by it and flows out through apertures between the vanes. Thus, the fluid undergoes a change in direction and is accelerated. This produces an increase in the pressure at the pump outlet. When leaving the impeller, the fluid may first pass through a ring of fixed vanes which surround the impeller and is commonly referred to as a diffuser. In this device, with gradually widening passages, the velocity of the liquid is reduced, its kinetic energy being converted into pressure energy. Of course it is noted that in some centrifugal pumps there is no diffuser and the fluid passes directly from the impeller to the volute. The volute is a gradual widening of the spiral casing of the pump. Centrifugal pumps are well known and are widely used in many different environments and applications.
The prior art also refers to centrifugal pumps as velocity machines because the pumping action requires first, the production of the liquid velocity; second, the conversion of the velocity head to a pressure head. The velocity is given by the rotating impeller, the conversion accomplished by diffusing guide vanes in the turbine type and in the volute case surrounding the impeller in the volute type pump. With a few exceptions, all single stage pumps are normally of the volute type. Specific speed Ns of the centrifugal pump is NQxc2xd/Hxc2xe. Ordinarily, N is expressed in rotations per minute, Q in gallons per minute and head (H) in feet. The specific speed of an impeller is an index to its type. Impellers for high heads usually have low specific speeds, while those for low heads have high specific speeds. The specific speed is a valuable index in determining the maximum suction head that may be employed without the danger of cavitation or vibration, both of which adversely effect capacity and efficiency. Operating points of centrifugal pumps are extremely important.
There are several common methods to identify the pressure at the suction side of a centrifugal pump. One common technique is the use of any type of pressure measurement device, which would include pressure transmitters, pressure transducers, bourdon tube gages and manometers. These are connected directly to the suction pipe near the pump and therefore measure the suction pressure. Certain pumps which are installed at the outlet side of vented tanks for the purpose of controlling tank level have their suction pressure calculated using the level in the tank. For example, knowing the pump""s hydraulic characteristics along with its actual speed, flow and discharge pressure, the suction pressure is and can be calculated.
Essentially, one can monitor the level in the tank to determine how fast the level goes down and how fast the level goes up and by taking various measurements to determine the suction pressure of the pump. While these devices are relatively widely employed, direct pressure measurement at the suction inlet of a centrifugal pump is the most accurate and direct measurement that is presently employed. The more serious drawback is that the approach requires a breach of the pumped suction pipe. Where the pumpage is highly flammable, caustic or environmentally dangerous, this could be a tremendous detriment. In this manner, once there is a breach of the suction pipe, the unit, which is normally connected to an electrical source, can cause ignition or combustion of the pumpage material and so on.
Calculating the suction pressure of a pump using the upstream tank level is not as accurate as direct pressure measurement. At high flow rates, the friction losses are not taken into consideration. There are a lot of changes in pumpage temperature and specific gravity and these changes also result in errors in calculation. The method of using the pump""s hydraulic characteristic is an indirect one. This method requires the use of a pump discharge pressure transmitter, a flow meter and a speed sensor. Additionally, the hydraulic performance of the pump has to be known. Specific gravity changes in the pumped fluid will result in errors. Net velocity head corrections to the total dynamic head (TDH) of the pump requires additional information and calculations. The approach assumes that the performance of the pump is in total agreement with the hydraulic data sheet. Unfortunately, often this is not true.
The present invention describes a new method and apparatus which eliminates many of the shortcomings of the prior art devices. The present method does not require any traditional instruments, and does not require a breach of the suction piping to directly measure pressure. The technique to be described is not effected by line losses and there is no need to know the pump""s hydraulic performance.
The present invention requires the use of a variable speed drive (VSD) for the pump motor. The drive utilized has the ability to characterize the motor to obtain torque supplied by the motor and the actual motor running speed. This feature, is provided in most variable frequency drives as presently implemented in today""s technology. The present invention requires that the pump discharge pressure and torque be measured at at least two different speeds. Discharge pressure is plotted versus torque. A first order curve (line) is fitted through the two points and where the line crosses the discharge pressure (y-axis) determines the suction pressure value.
As indicated, the above invention can be used on any centrifugal pump where the torque applied to the pump and the pump speed and pump discharge pressure is known. This can also be accomplished by the use of a torque shaft between the motor and pump and a pump discharge pressure transducer. Most torque shafts have apparatuses providing the ability to measure speed. Driving the pump is a variable frequency drive (VFD) and the pump discharge pressure transducer. As indicated, VFDs built today can characterize the motor and calculate both the torque generated by the motor and the actual speed of the motor.
The pump discharge pressure must be measured with an absolute pressure sensor, with a gage pressure sensor is used some barometric pressure sensor, indicator input needs to be employed. As indicated, the invention requires that pump discharge pressure and torque be measured at two different speeds. Discharge pressure is plotted against torque. This produces a first order curve or line which is fitted through the two points plotted. Where this line crosses the discharge pressure, is the value through a pump suction pressure. This will be explained in conjunction with the Detailed Description.